Over the last few decades, I've worked in several different gun shops and a pawn shop that did a huge amount of gun-related business. This response is based on my experience working in those shops.
First, any shop that takes trades only applies a percentage of the value to the trade. I don't have any idea of what your weapon might be worth, so I'll just throw out some nice round figures to keep things simple. Let's say the "book value" of your trade gun is around $1000. Most dealers will only give you a portion of the appraised value in trade; depending on the weapon and how hard or easy they think it will be to sell, that value might be between 50% and 65% (perhaps as high as 75% if they already have a buyer in line, looking for that exact model). So right up front, you will only be getting $500 to $650 trade value for your $1000 gun. And this is assuming your gun is absolutely New-In-Box. Lord help you if it has a nick on the grips, any bluing wear on the sharp edges of the rib, or the mag has been slid in and out a half-dozen times, rubbing a line on it; then you'll spend an hour arguing whether the gun is in 99%, 98%, or 95% condition, as that will make a HUGE difference in the book value.
When you trade, you'll be trading against the full sticker price of the gun you want to buy/trade for. A SIG 239 goes for about $550-$850 new, depending on the area of the country, caliber and options. So you can see, based on the mythical $1000 weapon I started with, that if you walked into a shop with a thousand-dollar gun and asked what the trade terms would be for the less-than-desirable-caliber $550 SIG 239 under the glass counter, you might be told that straight across is the best they could do. If the SIG had night sights, a special trigger, or other extra-cost option(s), then you'd have to give up your $1000 gun AND throw in some more money to make it happen.
Again, I have no idea of the objective value of your weapon. But unless it's a weapon that is easy to move (read: in great and steady demand, and not just by a few collectors or single-niche target shooters), be prepared for a shock when you ask about trades. If the dealer thinks he might have to sit on it for a while until it sells, the trade value will go down even further. On the other hand, a private sale (no dealer involved, if legal in your state/locality) is ALWAYS a better deal for the seller, as he gets to collect/keep the full dollar value for the weapon. The seller ALSO gets to put up with ridiculous low-ball cash offers, alternative trade offers, people trooping through his house/business, arguments about the condition of the weapon and whether it's original/reblued/unmodified, scumbags who can't buy a weapon from a store that does background checks, calls at all hours of the day and night, calls for weeks/months after the gun is sold, and possible targeting by thieves if they can find out where the weapon is stored or they think they can hold you up when you meet to sell it. This why internet auction sites are popular nowadays, and (in the olden days) consignment sale arrangements at gun shops were common and oft used; they get around most (but not all) of the negatives listed above.
Personally, to get two guns AND cash out of a trade, your gun will have to be worth an absolute BUTTLOAD of money, and you're STILL going to take a bath when you do the deal. So take your tranquilizers and/or blood pressure meds before you go.
Sorry to be the bearer of bad news, if this is the first time you've heard it.